The quadrivalent human papillomavirus (HPV) vaccine, called Gardasil has been on the market since 2006. This vaccine has been very effective in preventing cervical cancer as well as genital warts which is quite prevalent in sexually active teens. The recommended immunization schedule is to receive your first dose at age 10-11, followed by a second shot in 2 months from the first and then a third after 6 months.
Even though HPV vaccines have been considered very effective, fewer than one third of U.S. teens receive all 3 vaccines according to the Centers for Disease Control and Prevention’s National Immunization Survey of Teens. Many teens have only received 2 vaccines and doctors have wondered whether a different dosing schedule might save time and money while still providing protection.
Researchers in Canada did a study where Gardasil vaccines were delivered through school-based programs assigning 520 girls randomly (ages 9-13) to receive 2 or 3 vaccine doses of Gardasil. The two dose schedule was 6 months apart, whereas the girls who took the 3 doses got the vaccines on the regular schedule.
Researchers took blood samples to measure the number of antibodies made against each viral strain. They did this over time to see how long the antibody response would last. The girls who had 2 doses of vaccine made at least as many antibodies against each viral strain as the girls who did the 3 dose regimen. Also their protection appeared to last as long as those who had 3 vaccines.
One of the researchers proposed that the results of this study could raise the possibility that 2 doses could be given during pre-adolescence and the third dose later when girls are closer to the time when they are most likely to need HPV protection.
A more recent study from Sweden investigated the effectiveness of 1, 2 or 3 doses of the HPV vaccine for prevention of genital warts in girls and women (average age 10-24 years). Over 1 million women were followed over an average of 3.8 years. Compared with risk for genital warts in unvaccinated women, the risk was 80% lower in women who received all 3 vaccines and 70% lower in those who received 2 vaccines. The risk reduction varied with age in those that received only 1 vaccine. The risk for developing genital warts was 30% lower in those vaccinated at age 17-19 as opposed to 70% for those vaccinated before age 17.
We are still not sure how many years the vaccine is effective and whether 3 doses might protect women longer than 2 doses. But this large population study gives us some reassurance that women receiving 2 doses of HPV vaccine most likely have significant protection against HPV infection and development of genital warts. More complete protection may be provided with 3 doses on the currently recommended schedule.
References: Goodman, B. “Are 2 HPV Shots as Good as 3 For Preteen Girls?” WebMD News. April 30, 2014.
Wald, A. “Do We Need All 3 Doses of HPV Vaccine to Protect Against Genital Warts?” NEJM Journal Watch: Women’s Health. June 2014. Vol. 19 No. 6.